Estrogen receptor (ER) functions as a ligand-activated transcription factor for estrogen-regulated genes. Because of the critical role of the ER in the proliferation of certain estrogen-dependent cancer cell types such as the mammary tumor, inhibitors of estrogen action at the level of receptor function are of major clinical interest.
Estrogens are essential not only for the regulation of female reproductive functions, but also play critical roles in the propagation of a number of tumor phenotypes of estrogen target organs, such as the mammary gland. All of these hormonal functions are mediated through ERs, i.e. ERα, Erβ2. Among these receptor subtypes, ERα provides the dominant regulatory role in most target tissues. Primarily because of the important clinical use in the management of estrogen-dependent cancers, the search for an improved inhibitor of estrogen action has always been of significant endocrinological interest. Historically, estrogen analogs that bind to the receptor, but do not promote coactivator association and block trans-activation function, received major attention. These efforts have led to the development of several important antiestrogens, such as tamoxifen, raloxifene, and ICI 182780, all of which are used as pharmacological inhibitors of estrogen action. However, some of these antagonists also act as partial agonists, and most of them when used for a prolonged period give rise to drug resistance, possibly due to progressively increased metabolic inactivation. Additionally, the potential role of the ligand-independent constitutively active mutant forms of ER in the proliferation of certain types of cancer cells has been reported.
Alternate strategies for inhibition of ER function include a nonconventional molecular approach involving targeted overexpression of a dominant negative form of ER. This approach is based on the principle that a defective form of ER that can dimerize with the wild-type natural subunit will, upon overexpression in sufficient amounts, disable enough normal subunits and thereby inhibit the estrogen signaling cascade. The success of this approach for its therapeutic application may be dependent on massive overexpression of the defective subunit sufficient for inactivation of the wild-type receptor below a critical threshold level. Another approach is the selective intracellular destruction of ER mRNAs in target cells. Antisense ER transcripts can potentially function in this manner, and a major improvement in the antisense approach is achieved when the antisense specificity is combined with catalytic cleavage of the phosphodiester bond of the RNA target. Earlier studies with estrogen antagonists such as tamoxifen and ICI 182780 have shown that even at concentrations of 10- to 100-fold molar excesses over estradiol, these compounds can cause more than 50% inhibition of ERE-TK-Luc trans-activation and MCF-7 cell cycling. In the transient transfection assay, the ER-specific ribozymes and the hERα expression vector, only at an equimolar ratio, resulted in about 80% inhibition of ERE-TK-Luc trans-activation. Additionally, both of these potent antiestrogens have unique disadvantages, such as differential effects on target genes, the need for systemic administration, and the development of drug resistance after prolonged use. A need continues to exist for treatments that at least reduce and/or avoid such problems altogether in breast cancer therapies. An effective gene therapy approach for such a treatment has not yet been devised. At least in theory, such a therapy would potentially provide a targeted tissue-specific delivery of a ribozyme expression vector.